Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $31.35   6.491%   $33.3849   $33.38   $33.40   $33.40  
  Purchase #2 $248.48   3.569%   $257.3483   $257.35   $257.35   $257.30  
  Purchase #3 $120.42   8.488%   $130.6412   $130.64   $130.65   $130.60  
  Purchase #4 $237.85   8.115%   $257.1515   $257.15   $257.15   $257.20  
  Purchase #5 $161.69   11.268%   $179.9092   $179.91   $179.90   $179.90  
  Purchase #6 $71.35   7.595%   $76.769   $76.77   $76.75   $76.80  
  Purchase #7 $92.46   9.222%   $100.9867   $100.99   $101.00   $101.00  
  7 purchase totals = $963.60       $1,036.1908   $1,036.19   $1,036.20   $1,036.20